1. Field of Industrial Utility
The present invention relates to a rolling bearing, more particularly to the improvement of the life of rolling bearings that are to be used in transmissions, engines, etc., of automobiles, agricultural machines, construction machines, as well as iron- and steel-making machines.
The present invention further relates to rolling bearings, and more particularly to an improvement in service life of rolling bearings which are used, for instance, in the transmissions or engines of automobiles, agricultural machines, constructing machines or iron and steel machines.
2. Prior Art
It is well known that foreign materials such as metal cuttings, shavings, burrs and worn particles that get into lubricants for rolling bearings will damage their races and rollers, thereby causing a substantial reduction in their service life. Under the circumstances, the assignee previously filed U.S. Pat. No. 4,904,094, in which they proposed the following: if the contents of carbon, residual austenite and carbonitrides in the rolling surface layer of a rolling bearing are specified to appropriate levels, the stress concentration that will develop at edges of indentation due to a foreign material present in the lubricant for the bearing can be reduced and the occurrence of cracking can also be suppressed to improve the life of the bearing. According to this proposal, the life of the rolling bearing under lubrication in the presence of a foreign material can be improved by insuring that residual austenite will be present in an appropriate amount. On the other hand, the proposal involves the disadvantage that the residual austenite lowers the surface hardness of the bearing, thereby reducing its fatigue resistance. In addition, the proposal did not take into account the effects on the bearing's life of the particle size of carbides and carbonitrides; in particular, large-size carbides, if they are subjected to cyclic stress, will be the start point of fatigue, leading to the occurrence of cracking and flaking but this possibility was not considered by the above noted proposal.
However, one of the factors which shorten the service life of a rolling bearing is foreign matter mixed with the bearing lubricant. It is well known in the art that metal chips, shavings, burrs and/or powder are often mixed with the bearing lubricant. If the rolling bearing is used with the bearing lubricant containing such foreign matter, then the foreign matter may damage the track rings, namely, the inner and outer races, and the rolling elements of the bearing, thus reducing the service life of the latter. At worst, the service life is reduced to 1/10 of that which the rolling bearing has when the bearing lubricant contains no foreign matter.
In order to overcome this difficulty, the present Applicant has proposed a rolling bearing under U.S. Pat. No. 5,137,375. In the rolling bearing, the relationships between the amount of residual austenite (.gamma..sub.R vol %) and the hardness (Hv) of the surface layer of the rolling bearing are suitably determined that, even when a rolling bearing is used with the bearing lubricant containing foreign matter, concentration of stress at the edges of indentations formed by the foreign matter is lessened, and therefore the bearing is prevented from being cracked. That is, the rolling bearing is improved in service life.
The relationship of hardness between the track rings and the rolling elements of a rolling bearing have been disclosed by E.V.ZARETSKEY "A study of residual stress in during rolling", transactions of the ASME, April 1969. That is, it has been known in the art that the service life of a rolling bearing is increased when the hardness of the rolling elements is made higher by H.sub.R C 1 to 2 (in a C-scale of Rockwell Hardness) than that of the track rings, because residual compressive stress is produced as the rolling elements roll.
The rolling bearing disclosed by the aforementioned U.S. Pat. No. 5,137,375 suffers from the following problems or difficulties: In the rolling bearing, the amounts of residual austenite (.gamma..sub.R vol %) of the rolling surface layers of the track rings and the rolling elements are set in a range of 25 to 40 vol % by carburizing or carbonitriding them. In particular, as for the rolling elements, it is necessary to employed a high level heat treatment.
In addition, in order to suitably determine the relationships between the amount of residual austenite (.gamma..sub.R vol %) and the hardness (Hv), it is necessary to use a special heat treatment technique, which takes a long period of time to achieve the heat. Hence, the rolling bearing is low in productivity.
The conventional rolling bearing, in which the hardness of the rolling elements is made higher by H.sub.R C 1 to 2 than that of the track rings, has been proposed for the purpose of increasing the service life with the lubricant containing no foreign matters. That is, in the case of the conventional rolling bearing, no attention is paid to increasing the service life when it is used with the lubricant containing foreign matter.